The inventive concepts relate to a member for a gas sensor, a gas sensor using the same, and a manufacturing method thereof. More particularly, the inventive concepts relate to a nanoparticle catalyst-metal oxide nanofiber complex obtained by synthesizing an alloy nanoparticle within an apo-ferritin protein shell and functionalizing the alloy nanoparticle in the inside and on a surface of metal oxide semiconductor nanofibers, a member for a gas sensor using the same, a gas sensor using the same, and a manufacturing method thereof.
A metal oxide semiconductor-based gas sensors use a phenomenon that an electrical resistance value is varied by surface reaction occurring in a process of adsorbing and desorbing a specific kind of gas molecules on and from the surface of metal oxide semiconductor sensing materials.
The metal oxide semiconductor-based resistance variable gas sensors use a principle that a concentration of a gas is quantitatively detected by analyzing a ratio (Rgas/Rair) of a resistance (Rgas) in the specific gas to a resistance (Rair) in air, so constituents of a sensor system may be simplified and a size of the sensor system may be reduced. In addition, since various kinds of sensor arrays are manufactured at relatively low costs, the resistance variable gas sensors are widely used in various fields such as a harmful gas leak alarm, an air pollution measuring instrument, an alcohol detector, and a fire alarm.
Recently, various researches have been conducted for an exhaled breath sensor that accurately detects a very small amount of a biomarker in exhaled breath to early diagnosis of a specific disease in the human body. Specific metabolites are occurred during metabolism of disease factors in the body. These metabolites may be used as a biomarker representing the specific disease. Most of these metabolites may be in a volatile organic compound gas state, so a very small amount of these materials may be exhausted by the exhaled breath through the lungs. Acetone (CH3COCH3), toluene (C6H5CH3), ammonia (NH3), nitrogen monoxide (NO), and hydrogen sulfide (H2S) correspond to representative biomarkers in the exhaled breath and are known as gases related to diabetes, lung disease, kidney disease, asthma, and foul breath, respectively.
As awareness of a health issue becomes higher, there is requirement of a sensor technique capable of rapidly detecting of a very low concentration of harmful environmental gases to the human body or a high-sensitivity, high-selectivity and high-response sensor technique capable of early monitoring whether the human body is abnormal or not. Conventional metal oxide semiconductor-based gas sensors may have a long response time and a long recovery time of several seconds to several minutes. The response time may be a time for which the gas sensor responds to the gas, and the recovery time may be a time for which the gas sensor returns to the original condition. In addition, a performance of the conventional metal oxide semiconductor-based gas sensors may be rapidly varied according to humidity, pressure, temperature and atmosphere of the circumference. Furthermore, the conventional metal oxide semiconductor-based gas sensors may have poor selectivity with respect to a specific gas and may not have a limit of detection which is capable of measuring a gas having a very low concentration of several ppb (part per billion) to hundreds ppb. Thus, a sensing material for a super-sensitivity gas sensor should be developed to accurately detect a very small amount of gases included the exhaled breath of the human body using the metal oxide semiconductor-based gas sensors.
To manufacture a super-sensitivity metal oxide semiconductor-based gas sensor, various researches are conducted for synthesis of various nanostructure-based sensing materials including nanoparticles, nanowires and nanotubes and sensors using the same. Since these nanostructures have large surface areas responding to gases, gas sensing characteristics of the nanostructures may be increased. In addition, since the nanostructures have porous structures, the gases may be rapidly diffused into the sensing material to allow the gas sensor to respond to the gases at a very high speed.
In addition to the researches which synthesize the nanostructures to increase a specific surface and a porosity of the sensing materials, researches are also conducted for a method of developing a super-sensitivity sensing material by fastening metal or metal oxide catalyst particles to a sensing member in order to detect a very small amount (e.g., tens ppb) of a gas. In the case that the catalyst is used, selectivity and a sensing characteristic of the gas sensor may be improved by a chemical sensitization method increasing a concentration of adsorption ions (e.g., O−, O2 and O2−) using a metal catalyst (e.g., platinum (Pt) or gold (Au)), or an electronic sensitization method improving sensitivity based on an oxidation number variation of palladium (Pd) or silver (Ag) (e.g., an oxidation number variation generated during formation of PdO or Ag2O).
However, even though researches are continuously conducted for the super-sensitivity sensing materials using the nanostructure having the large specific surface and many pores and several kinds of nanoparticle catalyst, the gas sensor may not have a characteristic capable of accurately detecting the gas having a low concentration of hundreds ppb or less with a high response speed and a high recovery speed.
In method of synthesizing the sensing materials, a process of manufacturing the nanostructure and a process of forming the pores may be complicated and difficult. When the nanostructure is synthesized using a deposition method or a chemical growth method, the nanostructure may be formed through complex processes to cause high manufacture costs and difficulties of mass production.
In addition, it may be difficult to manufacture the metal or metal oxide catalyst having a size of several nanometers and to uniformly distribute the catalyst on an entire area of the sensing material. For example, if the metal catalyst is synthesized using a polyol process, catalyst particles may have relatively large sizes (e.g., 3 nm to 10 nm) and may easily aggregate to each other. Thus, it may be difficult to uniformly distribute the catalyst particles on the surface of the metal oxide semiconductor sensing materials.
New materials and processes should be developed to overcome the problems described above. For example, it may be required to develop a simple process capable of manufacturing the nanostructure. In addition, it may be required to develop functional nano catalysts capable of being uniformly distributed without aggregation during a high-temperature thermal treatment process necessary to synthesize the sensing materials. Moreover, it may be required to develop a process of uniformly fastening the functional nano catalysts to the sensing materials having the nanostructure. Furthermore, it may be required to develop a method of easily synthesizing, in bulk, a new super-sensitivity sensing material that overcomes limitations of conventional noble metal-based catalysts to maximize catalyst activation. It may also be required to apply the new super-sensitivity sensing material to a sensor that accurately and selectively detects a harmful environmental gas and various kinds of volatile organic compounds included in the exhaled breath. In particular, it is required to develop a new catalyst synthesis process method of easily manufacturing a nano alloy catalyst having a new composition beyond a conventional catalyst characteristic and of applying the nano alloy catalyst to a metal oxide nano structure to easily change sensitization degree of relative sensitivity according to whether the catalyst is included or not.